Can you put some reserves into a "lock box" type account that is your money but locked at escrow for up to a period of time.  

Personally I try to reserve my HELOC for emergency or unexpected costs during a project, so if you can try not to use all of it for the initial purchase and planned rehab.

My starting strategy is to do a few fix & flips to build some cash reserves up and then get into BRRRR in a couple of years. 

All monies from reservations we have get deposited into our account, and then at the end of the month we do monthly statements and pay our owners out of our account. 

Cash reserves? 

If he has the income to support a $550k mortgage, and a healthy cash reserve fund, he should invest the money.

Changes some of my answer below, but here would be most of the types of transactions you would have in these accounts and between them.Property transactions would be: all property specific rents and expenses, allocated expense transfer to the master LLC and transfers to master LLC bank account for profit AND/OR property management fee.Master LLC transactions would be: Shared expenses going out, reimbursements/transfers from properties for their allocated share of those expenses coming in, the transfers from your properties deemed as profit AND/OR property management fee and then transfers to your personal accounts deemed as owner distributions.Other transfers that could occur is if the master LLC needs to transfer to the properties for capital expenditures above and beyond the reserves you might leave in their accounts.

I'm curious how you handle situations where a guest wants to modify their reservation and leave early?

I've included an example below to help illustrate this.So different lenders have different rates (which do vary even for DSCR loans) but these are factors they all consider.See example below:DSCR < 1Principal + Interest = $1,700Taxes = $350, Insurance = $100, Association Dues = $50Total PITIA = $2200Rent = $2000DSCR = Rent/PITIA = 2000/2200 = 0.91Since the DSCR is 0.91, we know the expenses are greater than the income of the property.DSCR >1Principal + Interest = $1,500Taxes = $250, Insurance = $100, Association Dues = $25Total PITIA = $1875 Rent = $2300DSCR = Rent/PITIA = 2300/1875 = 1.23If a purchase, you also generally need reserves / savings to show you have 3-6 month payments of PITIA (principal / interest (mortgage payment), property taxes and insurance and HOA (if applicable).

It’s why you’ll often hear about keeping adequate reserves.